Let me not to the marriage of true minds"Sonnet 116," 1–8
Admit impediments. Love is not love
Which alters when it alteration finds,
Or bends with the remover to remove.
O no, it is an ever-fixèd mark
That looks on tempests and is never shaken;
It is the star to every wand'ring bark,
Whose worth's unknown, although his height be taken.
"The marriage of true minds" is a phrase both widely used and difficult to understand, at least in the way Shakespeare meant it. When we speak of a "marriage of the minds" we get around the problem of what "true" means. The notion of compatible intellects is certainly part of the original phrase, but in Shakespeare the word "marriage" is less neutral; he's speaking of a total relationship—both intellectual and erotic. "True minds" doesn't mean "authentic minds," but "faithful spirits"; "truth" in the Renaissance still had "fidelity" as one of its primary senses.
If you follow this point, you can see how Shakespeare gets on to the topic of constancy in love. Love really isn't love at all, he says, if it bends under circumstances or alters because the world around it is in a constant state of alteration. Love isn't an affair of convenience, but resolute, like "an ever-fixèd mark" (a seamark: some static object used to guide navigation). Like a seamark, love should weather all tempests without tribulation, serving like a star to wandering barks. The world and its lovers too are errant, and need stable principles for guidance; love is imagined as an almost external thing, beyond the shifting sensations of the lover.